CRTs as image display devices have been replaced by matrix-type display devices utilizing liquid crystal, EL emitters, plasma emitters, etc. In particular, liquid crystal display devices utilizing liquid crystal are used in various applications, such as portable TV sets, word processors, and personal computers.
As to image display devices used in the above equipments, minute and larger-sized devices are desired. In order to display minute images, it is necessary that picture elements constituting a matrix be very small in size and very great in number. However, an increase in the number of picture elements requires an increase in the number of bus lines that function as gate signal lines and source signal lines. The larger the number of bus lines, the more is a line breakdown likely to occur. Moreover, as display devices become larger in size, bus lines become larger in length. As such, it is now becoming increasingly difficult to fabricate break-free bus wiring. Meanwhile, in production of such an image display device, increasing non-defective products is important subject for a decreasing production cost.
In order to solve the above problems, there has been developed a matrix-type display device having a function to correct bus-line breaks (see Japanese Unexamined Patent Publication No. 3-23425/1991 (Tokukaihei 3-23425)).
The following describes the function to correct bus-line breaks in the matrix-type display device disclosed in the above Publication.
As shown in FIG. 4, in a display section 70 of a liquid crystal display device as the matrix-type display device, a signal which drives picture element TFTs 74 for picture elements 73 are outputted from a gate driving circuit 71 through gate signal lines 72.
Meanwhile, a video signal is sampled by sampling gates 77, which is controlled by a sampling signal circuit 76, and the sampled video signal is outputted to source signal lines 75. A picture element capacitance 78 is connected to a drain electrode of the picture element TFT 74, and an image is displayed by liquid crystal sealed between the drain electrode and a counter electrode, not shown.
In such a liquid crystal display device, in order to relieve breaking of source signal lines 75, a wiring 79 is formed so as to surround three sides of the display section 70.
The wiring 79 intersects the source signal lines 75 respectively on the signal input side (upper side seen from the drawing) of the source signal lines 75 and on the non-signal input side (lower side seen from the drawing) of the source signal lines 75.
If a breakdown of the source signal lines 75 occurs at a point "P" in the drawing, the line breakdown is corrected in the following manner.
The intersections "Q" and "R" of the wiring 79 and the source signal lines 75 are connected by irradiation of a laser. As a result, in a section below the point "P" seen from the drawing, since a source signal is inputted from the non-signal input side of the source signal line 75, a normal image can be displayed.
However, there arises the following problem in the above-mentioned conventional image display device when a breakdown of the source signal lines 75 is corrected by connecting up and down of the source signal lines 75 through the wiring 79 formed around the display section 70.
In other words, since the wiring 79 is placed along the display section 70, so the length of the wiring 79 becomes longer, and since the wiring 79 intersects a lot of source signal lines 75, a parasitic capacitance of the wiring 79 is much larger than of the source signal lines 75. Namely, its parasitic capacitance becomes as large as that in the case where a source signal line capacity for making signal holding properties stable is provided to source signal lines 75 in a general VGA (Video Graphics Alley)-class liquid crystal display device.
Therefore, in a corrected portion, delay and distortion of a signal occur and an offset amount due to feedthrough of the sampling gates 77 is different from in an uncorrected normal portion. As a result, there arises a problem that a line breakdown cannot be completely corrected.
This problem becomes more significant in a driver monolithic-type liquid crystal display device because a driving circuit 71, a signal circuit 76 and sampling gates 77 are arranged by using a TFT that has inferior driving capacity to on LSI.